Radium and radon measurements in seawater

The scintillation or Lucas cell (Lucas, 1957) is made by coating the inside of a Plexiglass, quartz or metal cell with silver-activated zinc sulphide (ZnS[Ag]). After transferring the Rn to the cell, it is stored for 1-2 h to allow Rn daughters, Po, Pb, and Po to equilibrate partially. Alpha decays from Rn, Po and Po cause emissions of photons from the ZnS[Ag]. These are converted into electrical signals using a photomultiplier tube (PMT) attached to the cell and routed to a counter. 

After the Rn measurement, the water sample in the same container may be used for Ra measurement by Rn emanation. In this case the container is sealed for several days to several weeks to allow Ra to generate a known activity of Rn. Then Rn is again 

Schlosser et al. (1984) modified this technique to make high precision measurements of Ra in seawater. They degassed the sample by boiling 14 L for 45 min and transferred the Rn into an activated charcoal trap at -78 °C. The charcoal trap was warmed to 450 °C and the Rn transferred into a proportional counter with a mixture of 90 % argon and 10 % methane. Details of the proportional counter and associated electronics are given in Schlosser et al. (1983). 

The calculation of the excess Rn activity of the sample must include (1) a decay correction from the time the sample was collected until the mid-point of the counting time, (2) the fraction of equilibrium attained with the Rn daughters ( Po, Pb, Bi) before counting, (3) the efficiency of the detector, (4) the background of the detector and (5) the blank associated with the sample container and extraction system. These calculations and the errors associated with the measurements have been discussed by Lucas and Woodward (1964), Sar-miento et al. (1976) and Key et al. (1979). The best precision (2o) obtained for the scintillation counting procedures is approximately 13 %. Schlosser et al. (1984) claimed a precision of 11 % for the proportional counting technique. 

Radium is removed from Mn-fibre by leaching with a mixture of hot hydroxylaminehy-drochloride and HO or HO alone. Leaching may be easily accomplished in a Soxhlet extraction apparatus (Section 13.2.3.2). Ihe extract is filtered and mixed with lOmL of saturated Ba(N03)2 solution followed by 25 mL of 7molfL H2SO4 to coprecipitate Ra with 

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